Mazda 6 Kombi next car – what do I have to consider on the used car market?

Hello together, so – my Zafira just blesses the temporal, a “new” used one has to come. Basic requirements: – Diesel and the huge dog box must fit in the trunk – in price there is not much since I got stuck in my search at the Mazda 6 (about 2004-2006), there is already under 2500, – with under 200,000 km. So from the principle I find the great, cockpit fits, in this blue metallic (?) (please don’t hit, I really didn’t hit so a lot of idea of cars and had the last years always 15-20 years old Opel) and black I find it really great. So from the outside. Test drive so far did not go because of plaster arm. Of the ads I have seen so far, I would now have the feeling that I would get the same money, which I would have to lay for a 01/02 Zafira with green badge, a more valuable car. What I also found “out” is that the Mazda 6 has probably quite a topic with rust. At the doors, below n on the sides and underbody (?). Can this be done yourself, if a used rust has points, and if so how? Spatula mass and varnish? And if so, to what degree rust? Then I have encountered relatively often advertisements with “motor damage” and “control chain torn”. Does this really happen so often or can one somehow avoid that? Is there anything to consider? So if I look at a used one, what do I have to pay attention to? What is important? Would make sense, w or what do I notice during a test drive that the control chain is already not good? And what do I notice that xyz is wrong and what would be “normal” for this car? He will certainly drive differently than a 17-year-old Opel Zafira. I would be happy if you could give me some tips. Thanks already!